AP Environmental Science

## Learning Outcomes

- Define **biodiversity** and distinguish its three main levels
- Explain the difference between **genetic diversity**, **species diversity**, and **habitat diversity**
- Identify how **species richness** and **species evenness** both affect species diversity
- Compare communities that have the same richness but different evenness
- Interpret a simple **Simpson’s Diversity Index** calculation
- Explain why higher biodiversity usually increases ecological system stability and resilience

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand levels of biodiversity, with a focus on the following syllabus points:

- Know that biodiversity includes variation **within species, between species, and among habitatss**
- Identify the three main levels of biodiversity: **genetic**, **species**, and **ecological** diversity
- Explain that **species diversity** includes both **species richness** and **species evenness**
- Understand that two communities can have the same number of species but different biodiversity if abundance is uneven
- Know that greater genetic diversity can improve a population’s ability to adapt to environmental change
- Understand that more diverse natural systemss are often more resilient to disturbance
- Be able to calculate or interpret a biodiversity measure such as **Simpson’s Diversity Index** when given a formula

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What are the three main levels of biodiversity?
2. How is species diversity different from species richness?
3. Why can two communities with the same number of species have different biodiversity?
4. How does genetic diversity help a population survive environmental change?
5. What does a high Simpson’s Diversity Index value usually mean if the formula used is `1 - [Σn(n-1) / N(N-1)]`?

## Introduction

Biodiversity is one of the core ideas in AP Environmental Science because it connects ecology, evolution, conservation, and human impacts. On the exam, students are often expected to move beyond the simple idea that biodiversity means “lots of species.” The more accurate idea is that biodiversity describes variation at more than one level of biological organization.

In APES, the key distinction is simple but very important: biodiversity can be studied **within species**, **between species**, and **among environmentss**. If you mix up those levels, you can lose easy marks. You also need to know that high biodiversity is not just a descriptive label. It helps explain why some natural systemss recover from stress more successfully than others.

> **Key Term: biodiversity**
> The variety of life in an area, including variation within species, between species, and among habitatss.

### Genetic diversity: variation within a species

**Genetic diversity** refers to variation in genes or alleles within a population or species. This is the biodiversity level that exists **inside one species**, not between different species. If all the members of a population are genetically very similar, that population may struggle when conditions change.

> **Key Term: genetic diversity**
> Variation in genes within a species or population.

Why does this matter? Because natural selection can only act on the variation that already exists, plus any new mutations that arise. If a disease appears, a drought lasts longer than usual, or temperatures shift, some individuals may have traits that help them survive. If the population has high genetic diversity, it is more likely that at least some individuals can tolerate the new conditions.

A classic APES way to think about this is resilience. Suppose a plant population contains individuals that differ in drought tolerance. During a dry period, the drought-tolerant individuals are more likely to survive and reproduce. If all individuals had nearly identical genes, the whole population might decline sharply.

A population bottleneck reduces genetic diversity. When a population drops to very low numbers after overhunting, disease, fire, or drought, many alleles are lost simply because the individuals carrying them die. Even if the population later recovers in numbers, it may still have low genetic variation.

The northern elephant seal is a useful example. Its population fell to about 30 in the 1890s due to overhunting. The species later recovered in size, but the severe bottleneck reduced its genetic diversity. That means large population size does not always equal high genetic diversity.

Genetic diversity is also why monocultures are risky. If a field is planted with nearly identical crops, one disease or pest can spread through the entire system. In contrast, greater genetic variation can make total crop failure less likely.

> **Test Tip:** If the question describes variation in traits like fur color, disease resistance, or drought tolerance **within one species**, the answer is usually **genetic diversity**, not species diversity.

### Species diversity: variation among species

**Species diversity** describes the variety of species in a community. This is the biodiversity level that exists **between species**. However, APES students often make one of the most common errors here: they treat species diversity as the same thing as species richness. It is not.

> **Key Term: species diversity**
> The variety of species in a community, taking into account both the number of species and their relative abundance.

Species diversity has two main components:

- **species richness**: how many different species are present
- **species evenness**: how evenly individuals are distributed among those species
  > **Key Term: species richness**
  > The number of different species present in an area.
  >
  > **Key Term: species evenness**
  > How evenly individuals are distributed among the species present.

A community with 10 species is richer than a community with 5 species. But if one of those 10 species makes up almost all individuals, then the community may have lower overall species diversity than another community where the 10 species are more evenly represented.

Consider two forests:

- Forest A has 10 tree species, but 90% of all trees belong to one species.
- Forest B also has 10 tree species, but each species has about the same number of trees.

The **species richness** is the same in both forests: 10 species. But Forest B has higher **species evenness**, so it has higher **species diversity** overall.

This matters ecologically. In a more even community, ecological community processes are less dependent on one dominant species. If one species declines, others may still maintain food web links, energy flow, or nutrient cycling. In a highly uneven system, the decline of the dominant species can be far more destabilizing.

This is one reason more diverse systems often recover more rapidly from disturbance. If a drought, storm, or disease affects one species, another may partly fill a similar ecological role. In a low-diversity system, there may be no backup.

You can see this in food webs. A community with many species usually has more feeding connections. That creates alternate pathways for energy flow. If one pathway weakens, another may still function. In a simple food chain, the loss of one species can collapse the whole chain.

> **Test Tip:** If a question says two communities have the same number of species, do not assume they have the same biodiversity. Check whether one community is dominated by one species. That is an **evenness** clue.

### Biodiversity diversity: variation among habitats and ecological processes

**ecological system diversity** refers to the variety of ecological communitiess, habitats, communities, and ecological processes in a region. This is the broadest APES biodiversity level. It is about differences **among ecological systemss**, not just counts of species.

> **Key Term: ecological system diversity**
> The variety of natural systemss, habitats, communities, and ecological processes in a region.

A region that includes forests, wetlands, rivers, grasslands, and estuaries has greater habitat type diversity than a region made up of only one habitat type. Each habitat type provides different abiotic conditions, different niches, and different species interactions.

Why does this increase biodiversity overall?

First, different habitatss support different species. A wetland can support amphibians, emergent plants, and waterfowl that would not survive in a grassland. A forest canopy provides habitats that open water does not. More habitat types usually mean more possible niches.

Second, habitat diversity supports a wider range of ecological processes. For example:

- wetlands filter water and slow flooding
- forests store carbon and regulate local climate
- streams transport nutrients and support aquatic food webs
- grasslands support grazing species and deep-rooted plants

These differences create a more varied regional environment and often allow greater total biodiversity.

This idea also helps explain why habitat fragmentation is so damaging. When development breaks a continuous habitat into small isolated patches, ecological variety diversity may decrease at the regional scale, and the amount of usable habitat for many species shrinks. Even if some patches remain, the ecological processes linking them may be weakened.

A useful APES example is a coastal region containing mangroves, estuaries, open ocean, and adjacent forest. That region has much higher habitat diversity than a simple coastline with only one dominant habitat. As a result, it can support more species and more ecological functions services.

Students sometimes confuse environmental system diversity with species diversity because both may involve “many kinds of life.” The difference is the level of focus. If the question is about many **habitats or environmental systemss**, think environments diversity. If it is about many **species in one community**, think species diversity.

### Measuring biodiversity: Simpson’s Diversity Index

APES sometimes includes a quantitative biodiversity measure, most commonly **Simpson’s Diversity Index**. This helps combine richness and evenness into one number.

> **Key Term: Simpson’s Diversity Index**
> A measure of biodiversity that uses species abundance data to estimate how diverse a community is.

A common APES form is:

$$
D = 1 - \frac{\Sigma n(n-1)}{N(N-1)}
$$

Where:

- $n$ = number of individuals of each species
- $N$ = total number of individuals of all species

In this version, values closer to **1** mean **greater diversity**, and values closer to **0** mean **lower diversity**.

Here is a worked example.

A pond community contains:

- Species A: 4 individuals
- Species B: 3 individuals
- Species C: 3 individuals

Step 1: Find $N$

$$
N = 4 + 3 + 3 = 10
$$

Step 2: Calculate $ n(n-1) $ for each species

- Species A: $ 4(4-1) = 4 \times 3 = 12 $
- Species B: $ 3(3-1) = 3 \times 2 = 6 $
- Species C: $ 3(3-1) = 3 \times 2 = 6 $

Step 3: Add them

$$
\Sigma n(n-1) = 12 + 6 + 6 = 24
$$

Step 4: Calculate $ N(N-1) $

$$
10(10-1) = 10 \times 9 = 90
$$

Step 5: Substitute into the formula

$$
D = 1 - \frac{24}{90}
$$

$$
D = 1 - 0.267
$$

$$
D \approx 0.733
$$

So this pond has a Simpson’s Diversity Index of about **0.73**, which indicates fairly high diversity.

Now compare that with another pond:

- Species A: 8 individuals
- Species B: 1 individual
- Species C: 1 individual

It has the same richness, because there are still 3 species, but the evenness is much lower. Its Simpson value would be lower too, showing lower species diversity.

Common student errors:

- confusing species richness with diversity
- forgetting to calculate total $N$ correctly
- using the wrong Simpson formula interpretation

Some classes use other versions, such as $ \Sigma(n/N)^2 $ or $1/D$. In some of those forms, higher numbers mean lower diversity. So always check the exact formula given.

> **Test Tip:** Before interpreting Simpson’s Index, check which version of the formula is being used. Do not assume “higher always means more diverse” unless the formula clearly supports that.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Biodiversity includes variation within species, between species, and among habitatss
- The three main APES levels are genetic diversity, species diversity, and ecological diversity
- Genetic diversity refers to variation in genes within a species or population
- Higher genetic diversity usually improves a population’s ability to adapt to change and resist disease
- Species diversity is not just the number of species; it includes both richness and evenness
- Species richness is the number of different species in an area
- Species evenness describes how evenly individuals are distributed among species
- Two communities can have the same richness but different diversity if one is dominated by one species
- natural systems diversity refers to the variety of habitats, biotic systemss, communities, and ecological processes in a region
- More biodiversity often makes natural systemss more stable and resilient to disturbance
- Population bottlenecks can reduce genetic diversity even if a population later recovers in size
- Simpson’s Diversity Index combines abundance data to estimate biodiversity, but the formula version must be checked before interpreting it

## Key Terms and Concepts

- biodiversity
- genetic diversity
- species diversity
- species richness
- species evenness
- ecological system diversity
- Simpson’s Diversity Index


Introduction to Biodiversity - Levels of biodiversity